We've all heard of black holes but are unlikely to see one (unless the Large Hadron Collider in Switzerland creates one that gobbles us all up). Blue holes, on the other hand, are dotted all over the planet, and available to dive into.
Submarine caves or sinkholes, they were created during the Ice Age, and their dark blue water forms a striking contrast to the land – or sea – around them. The best places to find them are the Bahamas and Belize.
The one pictured top is the Great Blue Hole at Ambergris Caye in Belize, while the one below that is Uncle Charlie's Blue Hole on the Bahamian island of Andros. Dean's Blue Hole (below) on nearby Long Island is the deepest at 663ft.
In addition to looking spectacular, blue holes offer up a fascinating world for scientists to explore. Because they are much deeper than the water (or land) around them, they have been left on their own for millions of years and have been largely unaltered by time. This means they have unique ecosystems that can teach explorers about events that happened thousands or more years ago. RM